configure.ac: use AC_PROG_CC_C99

Various parts of Xfce, and libraries used by Xfce, already use C99 features.
Therefore it is reasonable to expect the availibility of a capable compiler.
Current GCC releases silently default to newer C standards, starting with
gcc-5 which defaults to C11.

Replace AC_PROG_CC with AC_PROG_CC_C99 to switch the discovered, or specified,
compiler to C99.

Signed-off-by: Olaf Hering <olaf@aepfle.de>